Regional Anesthetics Flashcards
(38 cards)
What’s the difference between nociception and pain?
Pain = perception and awareness of noxious stimulus
Nociception = activation of nociceptors while unconscious
A nociceptive stimulus causes what 2 physiologic processes to occur?
- sympathetic stimulation - catecholamine release > incr HR/BP
- stress response - ACTH release > incr cortisol/BG
What are 3 reasons to provide regional anesthesia?
-
pre-emptive analgesia - abolish afferent nociception, reduction of anesthetic/analgesic drugs
- reduction of intraoperative complications
- better intra and postoperative pain control - faster return to normal activities
- prevents stress response - better immune system activity
What is an aminoester?
- hydrolyzed by cholinesterase enzyme
- causes anaphylactoid reactions
e.g. procaine, tetracaine, benzocaine, cocaine
What is an aminoamide?
- hepatic metabolism (microsomal enzymes)
- slow metabolism
- toxicity from accumulation is more likely
e.g. lidocaine, bupivacaine, ropivacaine, mepivacaine
Local anesthetics are essentially what two types of compounds linked together?
a lipophilic unit linked to a hydrophilic unit
What is the MOA of local anesthetics?
- Local anesthetic needs to penetrate into cell to produce its effects
- Blockade of Na+ channel
- Prevents Na+ influx > no depolarization > no impulse transmission
What are the lipid properties of local anesthetics?
- lipid solubility
- correlates w/ potency: more lipid soluble = more potent (ie. bupivicaine)
- facilitates penetration thru nn membranes
- promotes sequesteration into lipid-soluble compartments (ie. myelin) > slower onset + longer duration of action
What does pKa stand for and what properties does it instill for local anesthetics?
pKa = dissociation constant
- pH at which 50% of drug is present ionized (charged) and 50% is unionized (neutral)
- unionized = lipid soluble
- degree of ionization will depend on pKa and pH of tissue
- local anesthetics are weak bases > ionized in acidic pH
- local anesthetics are ion trapped in acidic environments
What happens with the local anesthetic effect for a drug like lidocaine with a pKa of 7.8 in infected tissue with a pH of 6.5?
Much more of the drug will be ionized when injected into the infected tissue, thus there will be fewer unionized liposoluble molecules available to have an anesthetic effect
What 5 things affect the effect of local anesthetics?
- pH/pKa
- proximity
- dose
- volume
- spread
What 6 things affect the onset of local anesthetics?
- lipophilicity
- pKa
- concentration
- dose + volume
- proximity to nerve
- type of nerve
What 5 things affect the duration of local anesthetics?
- vascular effect
- tissue blood flow
- vasoconstrictor
- dose
- affinity to the Na+ receptor
Smaller nerve fiber are ______ susceptible to local anesthetic b/c shorter length of axon is required to be blocked to halt the conduction completely
_______ fibers are more susceptible to be blocked b/c local anesthetic pools near the axonal membrane
more; myelinated
Nerve block onset occurs in what order?
- pain
- cold
- warm
- touch
- deep pressure (more evident in long acting LA)
- motor function
(Pain —–> motor function)
What is the clinical utility of knowing the order of senstations lost due to nerve block onset?
low concentrations of local anesthetic can provide analgesia with mild effects on motor function
What 3 things are associated with toxicity of local anesthetics?
- high dose (high plasmatic level)
- direct IV injection
- reduced biotransformation or elimination (liver/renal dz)
What are potential CNS signs of LA toxicity?
- onset before CV signs (except bupivacaine)
- depression of cortical inhibitory pathways
- impaired vision
- tremors/seizures
- CNS depression
- coma
What are the CV signs of LA toxicity?
- bradycardia
- incr PQ interval/QRS duration
- VPCs
- reduction of myocardial contractility
- CV collapse/cardiac arrest
What’s one way to ensure you don’t cause LA toxicity?
aspirate before injection to rule-out intravascular injection
What are the targets and basic principles behind topical/surface anesthesia?
- target structures: free nerve endings, nociceptors
- anesthesia for superifical mucosal structures
- generally LA cannot cross epidermis
What are some uses for topical/surface anesthesia?
- larynx - avoid laryngospasm during intubation
- splash block - local application before wound closure, reduce perioperative pain
- corneal - proparacaine drops - eye exam
- skin densensitization - EMLA cream (lidocaine + prilocaine) - venipuncture/small mass removal
What are the targets and basic principles behind local infiltration?
Targets: free nerve endings, nociceptors - does not target specific nerve
- anesthesia for skin and underlying structures
- when incisions are deep, all tissue layers are infiltrated
- injection of LA solution into/around planned sx field
What are some uses for local infiltration?
- resection of cutaneous/superficial masses
- sx closure of lacerations
- procedures involving an appendage
- line block - over/around incision line, SQ and underlying tissues have to be infiltrated
- intratesticular block - castration
- inverted L - flank laparotomies in ruminants - inj far from incision site, anatomy of incision line not altered, large vol of LA required
- ring block - around extremity, distal limb, tail, horn
